To reduce the noise produced by internal combustion engines, such as those used in automobiles and trucks, it is common to include a silencer or muffler in the exhaust line from the engine. In commercially available, dissipative type mufflers, exhaust gases from an engine are passed through or in close proximity to a mass of sound absorbing material. A commonly used sound absorbing material is fiberglass. The fiberglass may completely fill the shell or casing of the muffler or it may have a gas flow passage formed in it, which will reduce the back pressure caused by the muffler. One muffler design incorporating a sound absorbing material is described and illustrated in Paulsen U.S. Pat. No. 2,958,388.
Many commercially available mufflers for internal combustion engines utilize nondissipative sound attenuating structures and techniques. Rather than incorporating sound absorbing materials or structures, nondissipative or reactive mufflers incorporate and utilize acoustical side branch, wave cancellation and other nondissipative structures and techniques. The most simple design of a commercially available, nondissipative muffler consists of a perforated axial flow duct surrounded by a larger diameter casing or shell. The interior of the casing or shell is typically subdivided by transverse partitions into several compartments of different axial lengths. Sound attentuation is achieved by having the compartments of the shell act as acoustical side branches with respect to sound passing through the perforated flow duct into the compartments. Although such a straight-through, resonator type muffler is economical and produces a low back pressure, it also has a relatively low sound attenuation performance. One design of a straight-through, nondissipative muffler is described and illustrated in Marx U.S. Pat. No. 2,573,474.
Another common commercial design of a nondissipative muffler for an internal combustion engine utilizes a combination of acoustical side branches, wave concellation effects and other sound attenuating structures and techniques to provide a more effective performance than the straight-through type muffler. Such a higher performance muffler typically directs the exhaust gases from the internal combustion engine through a more tortuous flow path than the straight-through muffler before exhausting the gas. One such muffler which should offer better performance than a straight-through type muffler is described and illustrated in Noblitt et al U.S. Pat. No. 2,337,299.
As shown in the drawings for the patents described above, the shells, casings or outer walls for typical muffler designs are oval in transverse section. The oval shape is intended solely to minimize the amount of vertical space that the muffler will occupy beneath the body of a vehicle. By minimizing the vertically oriented dimension of the muffler, a greater clearance can be provided between the muffler and the road surface or, alternatively, the vehicle body may be lowered relative to the road surface. Although space and clearance considerations control the selection of the shape of the shell in typical, commercially available mufflers, it has also been recognized that particular geometric shapes for the shell or outer wall of a muffler may offer improved acoustical performance.
As examples of muffler designs that recognize the potential value of certain geometric shapes for muffler shells or outer walls, Rauen U.S. Pat. Nos. 2,138,510 and 2,274,459 describe and illustrate mufflers that incorporate axial flow conduits for exhaust gas and geometrically shaped side branches defined by the shells of the mufflers. The side branches have parabolic, hyperbolic, elliptical or spherical reflective wall surfaces. The reflective surfaces are oriented so that the geometric shapes are apparent only when taking axial or longitudinal sections through the mufflers. The geometric shapes of the reflective surfaces are selected so that sound energy entering the side branches may be reflected, focused and/or concentrated in one place until the energy is dissipated. At the same time, the exhaust gas introduced into each muffler is permitted to flow freely along the flow conduit and out of the muffler. In some of Rauen's mufflers, sound absorbing material, such as steel wool, mineral wood or asbestos, is used to fill the geometrically shaped, acoustical side branches.
Labussiere et al U.S. Pat. No. 3,692,141 describes and illustrates another muffling structure in which specific geometric shapes are utilized to attenuate noise emitted from a fluid flowing through a duct. In the Labussiere silencer, a baffle or air deflecting body is disposed in the middle of a flow duct from a jet propulsion unit, for example. The leading surfaces of the baffle or deflecting body are parabolically or elliptically shaped. The walls of the flow duct surrounding the body or baffle are lined with sound absorbing acoustical material. Thus, as gas flows through the flow duct and encounters the geometrically shaped surfaces of the baffle or deflecting body, sound travelling axially through the flow duct is reflected from the geometric surfaces into the sound absorbing material lining the walls of the duct. The gas, on the other hand, flows smoothly around the baffle or deflecting body to the exhaust of the duct.
Still another muffler or silencer for a gas flow is described and illustrated in Swiss Pat. No. 254,638. The Swiss silencer incorporates a housing or shell shaped as an ellipsoid of revolution or as a cylinder of elliptical cross section. A fluid inlet pipe opens into the housing at one focal point of its elliptical cross section. At the other focal point of the elliptical cross section is a sphere of material capable of intercepting sound waves reflected toward it. Surrounding the second focal point is a partition or baffle of semielliptical configuration which is parallel to and spaced from the housing. A narrow flow passage is thus defined between the inner surface of the housing and the outer surface of the partition, both of which surfaces are lined with sound absorbing material. The flow passage between the two surfaces leads to an outlet pipe from the housing, which is located, relative to the fluid inlet, behind the sphere of sound intercepting material and behind the partition. The silencer reduces sound levels primarily through the reflection of sound waves from and between the elliptical surfaces which are lined with sound absorbing material. By the time the fluid reaches the outlet from the silencer, the sound has had many contacts with the sound absorbing material within the housing and is significantly reduced in amplitude. At the same time, however, the tortuous flow path provided for the exhaust gas or other fluid tends to create substantial turbulence within the muffler and to increase the back pressure exerted by such a muffler, in comparision to axial flow type mufflers such as are commonly used in automobiles.